1. Field of the Invention
This invention relates to output driver circuits which employ MOS field effect transistors in a totem-pole or bridge connection for supplying power to a load device.
2. Description of the Prior Art
Totem-pole connections of MOSFET transistors have been used in output driver circuits to supply power to a load device. Each totem-pole circuit is formed by the series connection of a pair of MOSFETs. A bridge circuit can be formed by the combination of a pair of totem-pole drivers so that the load device can float between the outputs of the two totem-pole circuits and not require a direct connection to one of the supply terminals. These driver circuits find application in Pulse Width Modulation circuits where the desired low frequency signal is extracted from the PWM signal by a simple low pass filter network. These types of PWM circuits find application where high efficiency and low loss characteristics are desired for the driver circuit.
The simple totem-pole connection of MOSFET transistors unfortunately has catastrophic destructive modes for the MOSFETs. When supplying current to the low pass filter in a PWM circuit, or simply supplying current to an inductive load such as often encountered when these drivers are used for motor speed controllers, the commutating current flows through the alternate device's internal drain-source diode when the first MOSFET in the totem-pole pair turns off. When this first transistor returns to the on-state, the reapplied voltage, and high rate of change of current in the conducting drain-source diode form a set of destructive conditions.
The conventional solution to this problem is to provide an alternate path for the commutating current such that the current is prevented from flowing in the drain-source diodes of the MOSFETs. FIG. 2 shows the addition of 8 diodes to the bridge connection of MOSFETs where the diodes are configured such that commutating currents are prevented from flowing through the internal drain-source diodes of the MOSFETs. In FIG. 2 for example, if MOSFET devices 10 and 40 are initially conducting, forcing a current I (56) to flow through the low pass filter and the load as indicated, when devices 10 and 40 are turned off and forced into the non-conducting state, the presence of diodes 22 and 32 will prevent I (56) from flowing through the internal drain-source diodes of devices 20 and 30, and instead flow through diodes 24 and 34. This removes the set of destructive conditions from the MOSFETs and allows them to operate without overstress. An alternate connection for these diodes, and a more complete discussion of these protection circuits appears in the Motorola Data book Power MOSFET Transistor Data, Chapter 2-5, "Avalanche and dv/dt Limitations of the Power MOSFET," Q2/89, DL135, REV 3, to which the reader is referred for background and a detailed analysis.
There are several undesirable consequences of the use of external diodes to protect MOSFETs used in totem-pole and bridge circuits. The current handling capability of the external diodes has to be comparable to that of the MOSFET which is being protected. While diodes 12, 22, 32, and 42 can have very low breakdown voltage characteristic, diodes 14, 24, 34, and 44 must have comparable breakdown voltage characteristics to the MOSFETs being protected. These characteristics for the diodes result in component costs which add significantly to the total cost for the totem-pole or bridge circuit. Also, diodes with these characteristics are generally comparable in physical size to the MOSFETs, and therefore they consume an amount of printed circuit board that is comparable or larger in area than the area the MOSFETs themselves require. Finally, the voltage drop that occurs due to the diodes in series with the MOSFETs (12, 22, 32, and 42) leads to added dissipation in the totem-pole or bridge circuit and a corresponding reduction in efficiency. Removing the heat from these diodes can be difficult due to component and system packaging constraints.